Problem: Solve for $x$ and $y$ using substitution. ${-2x-y = -12}$ ${x = y+9}$
Explanation: Since $x$ has already been solved for, substitute $y+9$ for $x$ in the first equation. ${-2}{(y+9)}{- y = -12}$ Simplify and solve for $y$ $-2y-18 - y = -12$ $-3y-18 = -12$ $-3y-18{+18} = -12{+18}$ $-3y = 6$ $\dfrac{-3y}{{-3}} = \dfrac{6}{{-3}}$ ${y = -2}$ Now that you know ${y = -2}$ , plug it back into $\thinspace {x = y+9}\thinspace$ to find $x$ ${x = }{(-2)}{ + 9}$ ${x = 7}$ You can also plug ${y = -2}$ into $\thinspace {-2x-y = -12}\thinspace$ and get the same answer for $x$ : ${-2x - }{(-2)}{= -12}$ ${x = 7}$